Hormonal control of thermogenesis and energy dissipation.

Facultative (adaptive) thermogenesis is primarily controlled by the sympathetic nervous system (SNS). The participation of thyroid hormones in adaptive thermogenesis has been considered minor or, at most, permissive. The finding of type II-thyroxine (T(4)) 5'-deiodinase in brown adipose tissue (BAT) has opened a way to uncover a more important role for thyroid hormone in adaptive thermogenesis. This enzyme is activated by the. SNS and insulin. When activated, it generates high BAT concentrations of triiodothyronine (T(3)) from plasma T(4). T(3), intrinsically 10 times more active than T(4), has been shown essential for the expression of the key protein in BAT thermogenesis, uncoupling protein (UCP). The multihormonal control of BAT type-II 5'-deiodinase and the marked influence of T(3) on UCP and BAT thermogenesis suggest that the local control of T(3) generation may be an important source of variability in the potential of mammals to maintain temperature and dissipate energy.